Bats and giant bats both appear early in the game, and individually the giant bats are actually more dangerous. The danger with the bat is that it often brings a partner. Survive one bat and the second bat could finish you.

Bats have only a melee attack, and you have night vision of radius 1, so you can always see them when they attack you. Bats are infravisible, so races with infravision can even spot them through dark corridors. The bats of NetHack stay within melee range, making them easy targets for many players.

The reason why (real life) bats like the night is because it gives them cover from other creatures while the bats apply echolocation, employing sound to discover the positions of things. NetHack does not implement echolocation (though the biodiversity patch does, at least if you polymorph into a bat); in fact NetHack gives very little artificial intelligence to most monsters that do not use items and have only melee attacks, making them only approach the hero and whack them repeatedly.

Throwing something at a bat (even it if it is infravisible down the corridor) is most likely a waste of a turn if you happen to be unskilled or restricted in that weapon. Players who early wield an unskilled weapon to trade can encounter trouble. They think that they see an easy bat and try to kill it, only to miss too often and fear wasting a turn to wield a different weapon.

Alas, bats preexisted the #enhance command. Bats have annoyed adventurers seeking the Amulet of Yendor since the days of Hack, back when eating a dead bat would cause confusion because stun was not yet distinct.